Luminous Landscape Forum

Raw & Post Processing, Printing => Adobe Camera Raw Q&A => Topic started by: Rajan Parrikar on August 31, 2010, 03:41:29 pm

Title: CA correction question (ACR vs DXO)
Post by: Rajan Parrikar on August 31, 2010, 03:41:29 pm

Consider this image made with Canon 85L II lens @ f/1.2 -

(http://www.parrikar.org/images/LL/ca-example.jpg)

The lens correction setting for CA in ACR 6.2 does nothing to correct for the CA, shown inside the red circle on the image. (What kind of CA is this?)  

Whereas the following, processed via DXO, did help mitigate the CA -

http://www.parrikar.com/#/Goa/People/1


What special sauce does DXO have that ACR doesn't?

Title: Re: CA correction question (ACR vs DXO)
Post by: Ken Bennett on August 31, 2010, 06:09:13 pm

That looks like purple fringing, not chromatic aberration. I get that all the time on high contrast edges with the Canon fast primes shot close to wide open. In ACR try the Defringe drop down and see what happens, it can help some. I did find DxO did a nicer job with this issue.

Title: Re: CA correction question (ACR vs DXO)
Post by: Rajan Parrikar on August 31, 2010, 06:11:26 pm

Quote from: k bennett on August 31, 2010, 06:09:13 pm

That looks like purple fringing, not chromatic aberration. I get that all the time on high contrast edges with the Canon fast primes shot close to wide open. In ACR try the Defringe drop down and see what happens, it can help some. I did find DxO did a nicer job with this issue.

Yes, this is a wide aperture phenomenon on fast lenses.

I did try Defringe etc in ACR - nothing works, neither the Lens Profile nor the Manual options.

Title: Re: CA correction question (ACR vs DXO)
Post by: Rajan Parrikar on August 31, 2010, 09:02:50 pm

Update:  Following a tip, I went into ACR's HSL tab and dragged the Saturation slider of the Purples all the way down to zero.  Helps a lot!

(http://www.parrikar.org/images/LL/ca-corrected.jpg)

Title: Re: CA correction question (ACR vs DXO)
Post by: Bart_van_der_Wolf on September 01, 2010, 03:40:57 am

Quote from: Rajan Parrikar on August 31, 2010, 03:41:29 pm

The lens correction setting for CA in ACR 6.2 does nothing to correct for the CA, shown inside the red circle on the image. (What kind of CA is this?)

Hi Rajan,

It's longitudinal CA. You can see not only magenta 'fringes' in front of the plane of focus, but also green behind that plane (look at the top left). It shows most when (high)light areas are surrounded by dark areas. Overexposure makes it seem worse.

Longitudinal CA is the more difficult to correct than lateral CA, because it's size varies with distance from the focal plane instead of with radius from center, and it is not simple (although it can be done to some degree) to extract depth info from a 2 dimensional image. The simple approach is to watch out for overexposure, use a defringing type of correction, and selectively adjust saturation and/or color where it's most distracting.

The only way to really suppress it is by using a narrower aperture, on the 85mm f/1.2 it's almost gone at f/2-f2.8, but of course that will defeat the purpose of shallow DOF a bit (although at f/2.8 there is still little DOF when shooting close up). Here a full frame 24x36mm sensor also has a benefit because one can shoot from a shorter distance to fill the frame with the same crop as with a cropped sensor, which produces shallower DOF.

Cheers,
Bart

Title: Re: CA correction question (ACR vs DXO)
Post by: Stephen G on September 01, 2010, 03:00:15 pm

Quote from: Rajan Parrikar on August 31, 2010, 03:41:29 pm

What special sauce does DXO have that ACR doesn't?

DXO lens modules correct for longitudinal CA. ACR, as far as I know, does not. Check out the blurb on the DXO website:

http://www.dxo.com/intl/photo/dxo_optics_pro/optics_geometry_corrections/aberration (http://www.dxo.com/intl/photo/dxo_optics_pro/optics_geometry_corrections/aberration)

Title: Re: CA correction question (ACR vs DXO)
Post by: francois on September 08, 2010, 06:18:12 am

FWIW,
In the Lightroom 3.0 video tutorial, Jeff Schewe briefly mentionned that Adobe would take care of longitudinal CA in the future. While this doesn't give us any detail about "when" it's encouraging to hear.

Title: Re: CA correction question (ACR vs DXO)
Post by: madmanchan on September 09, 2010, 01:40:40 pm

Yes, this is longitudinal CA and is not corrected by ACR at present.

http://toothwalker.org/optics/chromatic.html